Numerous efforts have been made over the years to develop apparatus for making thin films for various applications. For example, U.S. Pat. No. 2,592,319 discloses a casting apparatus for film support or sheeting in which a layer of solvent based dope is cast onto the circumferential surface of a large, rotating dram or wheel. Just downstream of the hopper or die which casts the layer onto the drum, a fine meshed screen is positioned radially outboard of, but parallel to, the layer to prevent dryer gas currents from damaging the surface of the layer until the sufficient solvent has evaporated from the layer to enable the layer to resist such damage. A somewhat related technique is disclosed in U.S. Pat. No. 2,714,745 in which the casting wheel is provided just downstream of the hopper with a cover plate which defines a radially narrow plenum through which ambient gasses and solvent vapors are drawn back toward the hopper. The solvent rich gasses within the plenum prevent the dope leaving the hopper from setting prematurely and forming hardened slugs or strings on the surface of the cast layer. In such prior art dram casting apparatus, the common practice has been to circulate warn gasses over the surfaces of the drum to promote evaporation of the solvent. In some applications, it also is known to preferentially cool the edges of the drum surface not covered by the cast layer, to provide a more uniform temperature profile from the center of the layer to its edges.
In addition to drum or wheel casting apparatus of the types just mentioned, continuous or endless belt casting apparatus has been used. For example, U.S. Pat. No. 4,004,870 discloses a dual belt system in which a layer of solidifying resins is passed between a pair of endless belts, whose opposite sides are cooled by a liquid spray to promote setting of the resins. U.S. Pat. No. 4,584,231 discloses an apparatus for casting solvent based acrylic films, in which the endless belt passes through a series of heating and cooling zones which cause the solvent to evaporate from the film but are said to help prevent the formation of bubbles in the film. Another endless belt caster is disclosed in U.S. Pat. No. 5,075,064, in which flows of gas are directed against or parallel to both surfaces of the belt at a series of locations along the belt.
While apparatus of these types has achieved considerable acceptance over the years, a variety of problems have persisted. The flows of gas directed to the cast layer, particularly just downstream of the hopper, tend to cause surface irregularities such as ripples and similar defects which can render the resultant film useless for many applications, such as photographic products. After the flows of gas have been directed to the cast layer, the gas and evolved solvent vapor must be removed. Often, the evolved vapors and gas from the center of the cast layer pass over adjacent portions of the layer; so that, different parts of the layer are exposed to gasses having different concentrations of solvent vapor, which leads to non-uniform drying of the layer that can cause undesirable widthwise variations in the properties of the resultant film. As the solvent evaporates from the cast layer, the endless belt supporting the layer is cooled. Since the belt typically is significantly wider than the cast layer, the edges of the belt are not subjected to this evaporative cooling. As a result, the edges of the cast layer experience different drying conditions than the remainder of the layer, a situation which has been found to cause the edges of the layer to curl toward the longitudinal centerline of the cast layer after removal from the casting belt. This cud can make the web hard to convey through subsequent portions of the manufacturing equipment. The tendency toward edge curling can be reduced somewhat by reducing the operating speed of the belt, which tends to reduce manufacturing efficiency. Thus, while known belt casting apparatus is used rather widely, various problems tend to interfere with production of very high quality films such as are needed for photographic applications.